The present invention relates generally to extensometers, and more specifically to high temperature extensometers used, for example, for measuring high temperature creep of unidirectional composite materials.
Extensometers measure strain on a test specimen subjected to tensile, compression, fatigue or creep tests. Extensometers generally have a pair of arms connected together with some sort of spacing mechanism. The arms are coupled to move with the specimen during testing. A measuring device measures a change in distance between the arms in order to measure elongation or compression of the test specimen.
An example improvement to prior art extensometers may be found in U.S. Pat. No. 5,600,895 to Meyer et al. An example improvement specifically directed to high temperature testing may be found in U.S. Pat. No. 4,884,456 to Meline et al.
Most extensometers used for creep testing use strain gauges. Variations in strain readings, primarily drift, using such strain gauges have been a problem. Creep testing is typically performed over long time periods. Particularly at the low strain rates used for many composite creep tests, the strain readings over time are affected by many environmental factors. Electrical interference from the test system itself can cause a high frequency noise band of up to ±150 microstrain. Although electrical noise is not the source of the gradual changes in strain typically present in such creep tests, that noise is an indication of the limited range of strain gauge type extensometers. Such electrical noise may be reduced by additional filters or shielding.
The major source of strain variations over time during creep tests has been discovered to be the sensitivity of strain gauge type extensometers to changes in relative humidity. During a series of test at room temperature with the extensometer rods held in a fixed position as the relative humidity changed over time, the strain reading from the extensometer also changed up to ±150 microstrain. Even worse, in room air the changes in strain readings lagged behind humidity changes by 4 to 18 hours depending on the rate at which the humidity level changed. Placing an extensometer inside a vacuum chamber showed a further example of the effect of humidity. As air was evacuated from the chamber, the strain reading steadily decreased despite that the extensometer rods were held fixed. When room air was reintroduced into the chamber, the strain readings gradually returned to pretest levels.
The cause of the variation in strain readings was not determined, but may be due to the effects of moisture content on the strain gauges mounted on the extensometer or on the epoxy used to attach the strain gauges.
To minimize the effects of humidity on strain gauge readings, either the environment will have to be better controlled or another type of strain reading device not affected by humidity changes will have to be used. Attempting to better control the environment will be both expensive and introduce variations on its own, partly because the environment will be differently controlled at different test sites and at different times. A different type of strain reading device will more likely produce uniform, reproducible and comparable results at different times and locations.
Thus it is seen that there is a need for extensometers, particularly extensometers used in high temperature creep testing, that are nearly unaffected by changes in humidity, temperature and other environmental factors.
It is, therefore, a principal object of the present invention to provide an extensometer that is nearly unaffected by changes in humidity and other environmental factors.
It is an advantage of the present invention that it can be calibrated to be more sensitive to displacement than prior art extensometers while maintaining a more stable output.
It is a feature of the present invention that it is made using only one piece, providing the advantage of avoiding the errors in small-scale measurements from gaps at the interfaces between pieces.
It is another feature of the present invention that its physical geometry is such that all but one of the six degrees of freedom of movement are constrained, providing the advantage that only what is being measured can move.
It is another advantage of the present invention that its solid flexure configuration avoids hysteresis and non-linearity associated with moving mechanical connections.
These and other objects, features and advantages of the present invention will become apparent as the description of a representative embodiment proceeds.